Historically the frequency response of amplitude modulated (AM) radio receivers has been quite limited. In the United States, the Federal Communications Commission limits the AM bandwidth of a broadcast signal to about 5 kHz. Some AM receivers have even narrower bandwidths.
The narrow frequency response has contributed to a rise in FM broadcasting and a corresponding decline in AM radio. In order to counteract this decline, attempts have been made to improve the sound generated by an AM receiver. For example, for a number of years some AM broadcasting stations have been employing what is known as multiband "compression equalization" to improve the sound of the broadcast. Compression equalization involves amplitude compression of the higher amplitude signals, particularly at the upper end frequencies. It is believed that by decreasing the amplitude range between the higher amplitude and lower amplitude signals, the generated sound will appear to have an improved frequency range.
More recently, AM stereo broadcasting systems have been utilized in which two signals are transmitted on a single carrier. A compatible AM stereo receiver removes the audio signals from the carrier and generates left and right channel signals. A conventional AM stereo broadcast system is described in U.S. Pat. No. 4,218,586 by Parker et al, as well as U.S. Pat. No. 4,192,968 by Hilbert et al; both of these patents being incorporated herein by reference. In any AM stereo broadcast transmission, there is a 25 Hz subcarrier pilot signal present on the AM carrier signal. The presence of this pilot subcarrier notifies the AM stereo receiver that the signal is being broadcast in stereo in order to activate the proper decoding circuitry for generating the left and right channel stereo components.
It also has been desirable to maximize the frequency response of the AM receiver. Although the broadcasts are limited to a 5 kHz bandwidth, there are attenuated signals present which are above the 5 kHz corner frequency and which can be boosted after reception to improve the frequency response. It should be appreciated however, that utilization of improved audio components for reproducing these higher frequencies and for improving their fidelity is limited by the presence of noise, as well as the aforementioned stereo pilot signal and by the spectral distortion caused by compression equalization.
Additional problems associated with these wider band AM signals is the presence of 10 kHz beat notes which are generated by signals from two or more different stations having carriers which are 10 kHz apart. In a conventional AM receiver having a corner frequency of about 5 kHz, these beat notes were eliminated by notch filters having a fairly wide notch frequency. However, in a receiver having a frequency response above 10 kHz, this method of eliminating beat notes results in the loss of a significant portion of the frequency band.